1. Field of the Invention
This invention relates to electrical diodes, and more particularly to a heterojunction diode with a low turn-on voltage.
2. Description of the Related Art
While an "ideal" diode will become fully conductive under an extremely small forward bias voltage, practical diodes exhibit a significant forward-bias turn-on voltage. Diodes with smaller turn-on voltages would be highly desirable for applications such as compact, efficient power supplies for vehicles, satellites, radar and computer systems.
The diode that has exhibited the lowest turn-on voltage so far is a variation of the Schottky diode, in which a narrow layer of n+ GaAs is fabricated at the junction of a conventional metal/semiconductor device. This type of diode is described in Woodcock et al., "Control of the Height of Schottky Barriers on MBE GaAs", Electronics Letters, Vol. 19, No. 3, Feb. 3, 1983, pages 93-95, and Eglash et al., "Barrier Heights from Ohmic to Bandgap: Modified Al:GaAs Schottky Diodes by MBE", Proc., Int'l. Electron Devices Meeting, 1983, pages 119-122. The reported devices, with their associated turn-on voltages, include Au on GaAs (0.60 V), Al on GaAs (0.38 V), Ni on Si (0.36 V), Ni on GaAs (0.30 V) and Au on InP (turn-on voltage not reported, but energy barrier height was 0.50 eV, as compared to 0.57 eV for Ni on GaAs).
The modified Schottky diodes operate by reducing the differential between the conductive band energies of the metal and semiconductor at the junction (.phi..sub.B). This is illustrated in FIG. 1, which is an energy diagram in the vicinity of a junction 2 between a metal and a doped semiconductor, illustrated as n-doped GaAs. The semiconductor's conduction band energy E.sub.c peaks at the junction, with.phi..sub.B defined as the differential between E.sub.c and the metal's conduction band energy or Fermi energy, Eg. The references demonstrate that the insertion of a p+ layer on the semiconductor side of the junction can be used to increase the.phi..sub.B Schottky barrier height, and an n+ layer can be used to lower it. Reductions in the forward-bias turn-on voltage, down to a minimum of about 0.3 volts, can be accomplished in this manner. The n+ layer is formed from the same semiconductor as the n-doped material, and is simply given a heavier doping. While this reduction in turn-on voltage is helpful, still lower turn-on voltages would be highly desirable.
The Schottky diodes that were initially discussed above are presently used for high-speed power supply converters because they are majority carrier devices, with little minority carrier recombination. GaAs Schottky diodes have been found to offer the highest speed performance because of their transport properties. Although connecting multiple diodes in parallel to obtain a lower diode current density, with a correspondingly lower turn-on voltage, has been done in the past, this is not practical where circuit speed is a concern, since it also reduces the device's operating speed and prevents compact designs. System requirements for a diode with both a very low turn-on voltage and a high speed capability cannot be satisfied by available devices.
Various kinds of heterojunction devices have also been investigated over the past few decades. A heterojunction is a junction formed between two dissimilar semiconductors. Applications for heterojunctions that are of interest for the present invention include heterojunction bipolar transistors (HBTs), heterojunction field effect transistors (HJFETs), avalanche photodetectors (APDs) and lasers. Such devices are described in Ferry, ed., Gallium Arsenide Technology, Howard W. Sams & Co. 1985, pages 303-330 and 376-382, and Sze, Physics of Semiconductor Devices, 2d. Ed., John Wiley & Sons, 1981, pages 182-184, 350, 706-715, 763-765 and 780-783; a general heterojunction model is described in the Sze text at pages 122-129. Such devices are pertinent to the present invention in that they include heterojunction diodes imbedded within their overall structures.
A representative energy diagram for the emitter and base of a HBT is given in FIG. 2. The emitter is formed from a material having a relatively wide bandgap energy between its conduction and valence bands, while the base material has a narrower bandgap energy. The collector, which is not shown, can be formed from the same material as either the emitter or the base.
It is important that there be a close lattice matching between the emitter and base, to avoid a strain in the base that could otherwise generate lattice dislocations (crystalline defects). Such dislocations establish carrier recombination centers, and greatly reduce the device's gain. For emitter material such as AlGaAs or GaInP, a suitable lattice-matched base material is GaAs; for an InP or AlInAs emitter, a suitable lattice-matched base material is InGaAs. GaAs has a bandgap energy Eg of 1.4 eV, while InGaAs has an Eg of 0.75 eV. At these bandgap energy levels, the emitter-base junction would exhibit a forward-bias turn-on voltage greater than the 0.3 volts achieved with the modified Schottky diode described above, if it were isolated from the remainder of the HBT.
The generation of undesirable lattice dislocations is a function not only of the absolute lattice mismatch between the two materials, but also of the thickness of the material that is grown (typically epitaxially) upon the other material. As the thickness of the grown material increases, a lower lattice mismatch is required to generate dislocations. This phenomenon is illustrated in principle in FIG. 3, which presents a representative Matthews-Blakeslee curve comparing lattice mismatch with the thickness required to form dislocations. It can be seen that higher degrees of lattice mismatch can be tolerated if the grown layer is made thin enough. In the case of the HBT base material described above, for example, a lattice mismatch on the order of about 3% can be tolerated without the generation of dislocations in the base, but only if the base layer is restricted to a thickness on the order of tens of Angstroms. However, this is too thin for a practical HBT base. A base this thin would likely be fully depleted of charge carriers, without being able to establish the necessary charge-neutral region. Even if a charge-neutral region could be achieved, it would be so thin that its resistance would be too high. Thus, the choice of materials for the emitter-base junction in an HBT is quite restricted.
An example of an HJFET device would be one with an n-type InP channel and a p+ InGaAs gate; an energy diagram of the gate-channel junction is illustrated in FIG. 4. Since electron conduction is the dominant conduction mechanism, when considered as a diode in isolation the turn-on voltage of the gate-channel junction is determined by the bandgap energy of the conduction bands. The holes are typically avoided as the dominant conduction mechanism, since they are relatively slow. In.sub.0.53 Ga..sub.0.47 As is lattice matched with InP, and has a bandgap energy on the order of 0.75 eV. Again, if considered in isolation the gate-channel junction would have a forward-bias turn-on voltage considerably in excess of 0.3 volts.
Typical APDs have both a pn junction and a heterojunction, but they are at different locations within the device; the pn junction itself is a homojunction. However, "staircase" APDs do employ a pn heterojunction, typically with p-type AlGaAs on one side of the junction and n-type GaAs on the other; these materials are well lattice matched to each other. An energy diagram of the junction area is illustrated in FIG. 5. The lower bandgap material, GaAs, has a bandgap energy on the order of 1.4 eV. If the junction is considered in isolation, this would result in a turn-on voltage even greater than for the InGaAs mentioned above.
The case of a heterojunction laser is illustrated in the energy diagram of FIG. 6. An n-type active region 4 such as GaAs is formed between a p+ cladding layer 6 on one side and an n-type cladding layer 8 on the other side; both cladding layers are typically AlGaAs. The device is normally operated forward-biased, which causes electrons to be pumped from the cladding layer 8 into the active region 4 and holes to be pumped from the other cladding layer 6 into the active region 4. The electrons 10 and holes 12 are confined within the active region by the energy barriers on either side, and eventually combine to emit light. As with the other heterojunction devices described above, the pn junction between the cladding layer 6 and active region 4 would not exhibit a low forward-bias turn-on voltage, due to the relatively wide bandgap energy (about 0.7 eV) of the GaAs which is required for lattice-matching with the cladding layers.